Vehicle cooling systems vary widely in complexity, depending primarily upon the thermal requirements of the various vehicle systems employed in the vehicle in question. In general, these cooling systems utilize heat exchangers of one form or another to transfer the heat generated by the vehicle subsystems to the surrounding ambient environment. Such heat transfer may either be performed directly, for example in the case of a simple radiator coupled to a vehicle engine, or indirectly, for example in the case of a thermal management system utilizing multiple heat transfer circuits to transfer the heat through multiple stages in order to sufficiently lower the temperature of the component in question.
In general, vehicle heat exchangers are designed to exchange heat between two different fluids, or two similar fluids that are at different temperatures, thereby helping to maintain the various vehicle systems and components within a safe and effective operating range of temperatures. One of the fluids is typically composed of a refrigerant or water, the water often mixed with ethylene glycol or propylene glycol or a similar liquid that provides anti-freeze protection at low temperatures. In many vehicle heat exchangers such as condensers and radiators, the second fluid is air which is forced to flow through the heat exchanger, either as a result of vehicle movement or through the use of a fan.
Within the automotive industry there are several types of air heat exchangers, the design of each being based on their intended application. Exemplary heat exchangers include:                A powertrain radiator in which a coolant-to-air heat exchanger is used to remove heat from an internal combustion engine or electric motor.        A condenser in which a refrigerant-to-air heat exchanger is used to remove heat for cabin air conditioning systems or other systems (e.g., battery packs and power electronics) that employ refrigerant as the cooling fluid.        A transmission oil cooler in which an oil-to-air heat exchanger is used to remove heat from the transmission via the transmission fluid.        A steering pump oil cooler in which an oil-to-air heat exchanger is used to remove heat from the steering system via the steering fluid.        A charge air cooler in which an air-to-air heat exchanger is used to remove heat from turbocharged (compressed) air used in the engine intake system.        
For a given set of fluid temperatures, the performance of a fluid-to-fluid heat exchanger depends primarily on the surface area of the heat exchanger and the volume flow rate of the two fluids through the heat exchanger. Flow rate is commonly determined as the fluid velocity through the heat exchanger multiplied by the frontal area of the heat exchanger. Larger heat exchanger surface areas and mass flow rates result in greater heat transfer from the inner fluid to the outer fluid. An increase in these same variables, however, also results in an increase in the hydraulic losses, or pressure drop losses, which are manifested in increased aerodynamic drag (i.e., vehicle motive power), pump power, and fan power. Additionally, in a fluid-to-fluid heat exchanger, the transfer of heat between the two fluids increases as the temperature difference between the two fluids increases.
In a conventional vehicle utilizing multiple heat exchangers, regardless of whether the vehicle utilizes a combustion engine, an electric motor, or a combination of both (i.e., a hybrid), the individual heat exchangers are typically positioned one in front of the other, followed by a fan, this configuration referred to as a “stack”. In such a stacking arrangement, commonly the heat exchanger with the lowest outlet air temperature is located upstream, followed by higher temperature heat exchangers downstream. An example of such a configuration is a condenser followed directly by an engine radiator, followed by one or more fans. While this arrangement is more common with vehicles utilizing a combustion engine, hybrid vehicles may also use a stack of heat exchangers in order to provide cooling for the battery pack, power electronics and the motor. A principal drawback of the practice of stacking heat exchangers is an increase in hydraulic losses (i.e., fan power, aerodynamic drag) that result regardless of whether each heat exchanger in the stack is in active use. Additionally, since the temperature of the air entering the inner heat exchanger(s) will be the temperature of the air exiting the upstream heat exchanger which is typically higher than the ambient temperature, the efficiency and overall performance of the inner heat exchanger(s) is compromised. As a consequence, it is common practice to increase the surface area or thickness of the downstream heat exchangers to compensate for this decrease in expected performance which, in turn, adds weight and cost to the affected heat exchangers.
While a variety of different techniques and system configurations have been used to control the temperatures of the various subsystems and components in a vehicle, they are often inefficient, which in turn affects vehicle performance. Accordingly, what is needed is a thermal management system that achieves efficient heat transfer while effectively utilizing vehicle surfaces. The present invention provides such a thermal management system.